1. Field of the Invention
The present invention relates to a method of making electrodes for storage batteries of the lead-acid class.
Further, the invention relates to an improved electrode for storage batteries of the lead-acid class, and to an improved storage battery comprising such electrodes.
2. Description of the Prior Art
Electrodes for lead acid batteries are made essentially according to two principles, viz. pasted electrodes and tubular electrodes.
The first mentioned type comprises a grid which holds a dough (paste) of a mixture of lead oxides, water and sulphuric acid. If the electrode is intended to constitute a negative electrode (i.e. being used as the anode in the battery during discharge) also carbon black, barium sulphate and lignine are added. The dough is pasted into the grid, either manually or by means of any suitable equipment. The grid is cast in one piece of lead or a lead alloy and has the tasks of carrying the paste, which in the formed condition constitutes the active material, and conducting current to and from the active material. The grid may have different configurations to serve its purpose. Very often some of its spines are mutually somehwat displaced to enhance the ability of the grid to hold the active material. The grid also has a lug for the connection of other electrodes of the same kind in parallel, via a post strap, to a post, where a cable can be connected.
Tubular electrodes have a different grid design in that a number of straight and parallel spines are connected to an upper frame which has a lug. Also this kind of grid is cast in one piece. Porous tubes with a round, elliptic or square cross-section are thread on to the spines. To locate the spines centrally of each tube, the spines are provided with small protrusions or fins. The length of the tube is at least the same as that of the spine. The tubes are fixed to the grid only by pushing them on to the upper, conical parts of the spines. The application of the material, which after electro-chemical formation will constitute the positive active material, is made by turning the open ends of the tubes upwardly and filling a powder or a slurry of leady oxides into the tubes. Since the filling material is usually not a free flowing material, grid and tubes have to be vigorously shaken and vibrated during this operation. When the appropriate amount of material has been filled into the tubes, the ends have to be sealed by means of a bottom bar of plastic, which is pressed into the tubes. For the subsequent operation it is necessary that the bottom bar is well anchored. The tubes may be used either individually--one for each spine--or as a group of tubes, sewn or woven together. There could also be more than one spine in a tube.
Sometimes another element is added to the tubular electrode, viz. an enclosure of plastic arranged around the upper frame to protect it against corrosion. Such an arrangement also decreases the amount of antimony that can be dissolved from the frame. Furthermore, the portion of the current which passes from the upper frame to the negative electrodes is first forced through the positive active material, which means a higher current efficiency.
The porous tubes are made from braided or woven glass or polyester. Glass is very resistant to oxidation in the environment created by oxygen evolved on lead dioxide during charging, but must be supported and protected by a perforated sheet of e.g. PVC. Polyester tubes are impregnated by fenolic resin for the same reason. Very often the tubes are made from felted (non-woven) materials such as polyester. The properties of the tubing material shall be: high porosity to permit fast acid diffusion into the electrode and good mechanical strength to support the active material and retain it in the electrode.
The dough or powder applied to the grids and comprising lead compounds, must be transformed to lead dioxide in the positive electrode and to porous lead in the negative one by an electro-chemical process (formation) before the electrodes will be able to deliver any electricity. Before that, however, the pasted electrodes have to be cured, i.e. the paste must be dried in a special way so that no cracks are formed and so that a suitable mixture of lead compounds is formed from the paste. The metallic lead must also be oxidized in this process. The tubular electrodes have to be sprayed by or dipped into sulphuric acid ("sulfation") to bind the lead compounds and to let the acid react with the lead oxide in the filling material, whether it is applied as a dry powder or a slurry. If the material has been filled into the tubes as a granulated powder, which actually is free flowing, it probably already has been treated with sulphuric acid and the plates do not need to be processed further. The sulfation could last for between a few minutes to more than 20 hours, depending on the subsequent operations. Then, after a short rinse and drying, the electrodes are ready for the electro-chemical formation.
Characteristic for the properties of pasted electrodes is the relatively inexpensive way of manufacture, as pasting can proceed at a high rate. The electrodes are often made thin, especially if they are to be used in batteries for high current discharges. As there is no protecting cover on pasted electrodes, shedding of the active material will occur after a relatively short period of use. This is due to the expansion of the material itself and also due to the growth of the grid caused by anodic corrosion. By adding an extra separator--a retainer mat--when assembling the electrodes to a battery, the shedding can to some extent be delayed. Also the risk of handling the electrodes in production should be realized: the active material is not entirely bound at the curing process but has a strong tendency to give off leady dust.
By enclosing the active material in tubes and having the grid in the center of these tubes, a substantial increase of the battery life is obtained, because the active material is retained by the tubing and the conducting lead spine is much better protected from anodic corrosion than in pasted electrodes. Furthermore, a much higher porosity of the active material can be permitted, which increases the utilization of the material. The spine in the center of the tube will provide for an even current distribution at least in a horizontal cross section. No shielding effect can occur as is the case with pasted electrodes. The disadvantage of tubular electrodes, besides the extra cost for the tubing, is the high production cost. Usually, the tubes have a diameter of 6-10 mm, the spine has a diameter of about 3 mm and the length of the tubes to be filled can be more than 500 mm. The filling of these tubes through the open ends is a very slow process going on by heavy shaking and vibrations, which also causes an environmental hazard by the lead dust. After filling, the bottom bar shall be applied to 15-20 such tube openings simultaneously. It is also evident that it is difficult to fill the tubes evenly and to control attainment of a predetermined filling degree.
When the electrodes have been fully formed and dried they are assembled to batteries. It is also possible to assemble unformed plates and then form them in the battery in an acid having a lower concentration than the acid filled into batteries with already formed electrodes. A battery consists of one or several cell units, each one of which has a cell voltage of about 2.1 volt when no current is passing through the cell. The cell is built up of a number of positive and negative electrodes (the negative ones always being pasted electrodes) separated by microporous membranes of plastic, paper or glass wool) to prevent short circuits. The same kind of electrodes are connected in parallel and to a post strap and thereafter introduced into a cell jar. Mostly the outer electrodes are of the negative kind and thus a cell has one negative electrode more than it has positive ones.
The capacity which will be discharged from the battery depends on the magnitude of the discharge current, whereas the time during which this current can be drained from the battery depends on a lot of factors. Some factors could be considered as external, i.e. the ambient temperature and the voltage to which the battery shall be discharged without jeopardizing the function of the electric equipment it is supposed to serve. The internal factors are evidently the positive and negative active material and the amount of acid. It is important how these active materials are distributed and arranged in the cell, e.g. the porosity and the electrode thickness. A high porosity and a thin electrode usually give a higher capacity than a thick electrode with low porosity, but suffer from a shorter life. Some acid is located in the pores of the electrodes, which actually is the best place since acid must be available at the reaction sites. However, not all of the acid necessary to utilize the active material can be located in the pores. Therefore, most of the acid is disposed between the electrodes, in the separator and above the electrodes. (The acid below the plates does not contribute to the capacity unless there is a pump for acid circulation). Thus, the necessary amount of acid can be added to the cell by adjusting the mutual distance between the electrodes and by changing the height of the cell jar. The acid located outside the electrodes must be made available to the reaction sites by diffusion or migration to the interior of the electrodes. Thus, it will be realized that the electrodes must be thin and porous to allow for a fast transport of acid. Too much acid outside the electrodes is of no advantage, since not all of it can be used sufficiently, but will increase both the weight of the battery and the inner ohmic resistance.
Recently lead acid batteries have been designed to function as "sealed cells" which means that neither water decomposed to oxygen and hydrogen during charge nor water vapour can escape from the battery. The invention solves the problem of topping up i.e. adding water to the battery to replace what has been lost due to gassing and evaporation. The principle is to have oxygen, which starts to evolve at the positive electrode when about 80% of the discharged capacity has been recharged, transferred to the negative electrode where it is reduced back to water. This also means that hydrogen gas will not be evolved since normally that process does not occur until the negative electrode is fully recharged. It is, however, important for the function of a sealed lead acid battery that the distance between the positive and negative electrode is small for a fast transport of oxygen, which contradicts the demand of acid volume available for discharges. Especially the capacity at low currents where the active material has to be deeply discharged, is impaired. Sometimes the separator is not even filled with electrolyte in order to enable oxygen to flow freely to the negative electrode. The capacity of such cells with starved electrolyte is of course still lower. Organic materials cannot be used in sealed cells, at least not in contact with lead dioxide, due to the formation of CO.sub.2 which is not reacting in the cell and thus will increase the pressure. Therefore, the separators for sealed lead acid batteries are made from glass wool which also has a good wicking effect.